The present invention relates to retarders (preferably a phase retardation film) that are useful as xcex/4 plates in reflective liquid crystal display devices, pickups for writing on optical discs or anti-reflection coatings; circular polarizers using said retarders; and preparation processes thereof. Particularly, the present invention relates to retarders comprising a long transparent substrate and optically anisotropic layers having a laminate structure of rod-like liquid crystals formed thereon by coating; circular polarizers that can be prepared by laminating such a retarder to a polarizer in a roll-to-roll manner; and preparation processes thereof.
xcex/4 plates find great many applications and have already been practically used. However, most of xcex/4 plates achieve xcex/4 only at a specific wavelength though they are called xcex/4 plates. JP-A 1998-68816 and JP-A 1998-90521 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) disclose retarders obtained by laminating two optically anisotropic polymer films. In the retarder described in JP-A 1998-68816, a quarter-wave plate forming a birefringence light with a phase difference of a quarter wavelength and a half-wave plate forming a birefringence light with a phase difference of a half wavelength are laminated with their optic axes being crossed. In the retarder described in JP-A 1998-90521, at least two retarders having a retardation value of 160-320 nm are laminated at an angle with their slow axes being neither parallel nor orthogonal to each other. The retarders described in both documents specifically having laminate structures of two polymer films. Both documents explain that xcex/4 can be achieved in a wide wavelength region with this arrangement. However, the preparation processes of the retarders described in JP-A 1998-68816 and JP-A 1998-90521 require cutting two polymer films at a predetermined angle and laminating the resulting chips in order to control the optical directions (optic axes or slow axes) of the two polymer films. If retarders are to be prepared by laminating chips, the process becomes complex and other disadvantages occur such as liability to quality failure due to misalignment, decrease in yield, increase in cost and liability to deterioration due to contamination. Moreover, it is difficult to strictly control the retardation value in polymer films.
On the other hand, techniques for more easily providing a broadband xcexd/4 plate by including at least two optically anisotropic layers containing a liquid crystal compound are disclosed in JP-A2001-4837, JP-A2001-21720 and JP-A 2000-206331. Especially, the technique disclosed in JP-A 2001-4837, in which the same liquid crystal molecules can be used, is also attractive in terms of production costs.
During the preparation of a retarder by laminating optically anisotropic layers containing a liquid crystal compound, it is important to control the alignment of the liquid crystal compound. For example, optically anisotropic layers can be formed of homogenously aligned rod-like liquid crystal compounds with respect to a substrate. If a layer containing a rod-like liquid crystal compound is formed on an alignment layer subjected to a rubbing treatment, the rod-like liquid crystal compound is normally homogenously aligned with the long axis being in the rubbing direction. However, our careful studies revealed that the accuracy of liquid crystal alignment decreased when a conventional alignment layer is used for an alignment of liquid crystal molecules in a direction of the rubbing axis to form the liquid crystal molecular layer disclosed in JP-A 2001-4837. It was also found that many optical defects occurred with some types of rod-like liquid crystal compounds or with some types of liquid crystal compositions.
In view of the problems described above, the present invention aims to provide a retarder (preferably a phase retardation film) that can perform in a broad wavelength range (visible wavelength range) and can be easily and stably prepared in a thin-layer form. The present invention also aims to provide a circular polarizer that can perform in a broad wavelength range (visible wavelength range) and can be easily and stably prepared in a thin-layer form. The present invention also aims to provide a process by which a retarder and a circular polarizer performing well in a broad wavelength range (visible wavelength range) and capable of being in a thin-layer form can be prepared by stably controlling the alignment of the molecules of the liquid crystal compound. The present invention also aims to provide a novel technique for controlling the alignment of rod-like liquid crystals.
As a result of careful studies about retarders having laminate structures of xcex/4 liquid crystal layers and xcex/2 liquid crystal layers, we found that when an alignment layer which allows liquid crystal molecules to be aligned along the rubbing axis is used, the alignment of the liquid crystal molecules induced by the alignment layer is not stabilized though the rod-like liquid crystal compound should be homogenously aligned at a large azimuthal angle with respect to the longitudinal direction of the transparent substrate to form a xcex/2 liquid crystal layer. We also found that liquid crystal molecules are aligned in various directions near free surfaces to bring about optical defects in the liquid crystal molecules though the rod-like liquid crystals in the xcex/2 liquid crystal layer and xcex/4 liquid crystal layer should be homogenously aligned with high accuracy. Further profound studies based on these findings led us to accomplish the present invention.
In one aspect, the present invention provides a retarder (preferably a phase retardation film) comprising:
a long transparent substrate, and up on the substrate,
a first optically anisotropic layer formed from a homogenously aligned rod-like liquid crystal compound and having a phase shift substantially equal to n measured at a wavelength of 550 nm;
a second optically anisotropic layer formed form a homogenously aligned rod-like liquid crystal compound and having a phase shift substantially equal to xcfx80/2 measured at a wavelength of 550 nm; and
a first alignment layer having a rubbing axis defining the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the first optically anisotropic layer;
wherein the angle between the slow axis in the plane of the first optically anisotropic layer and the longitudinal direction of the transparent substrate is substantially 75xc2x0, the angle between the slow axis in the plane of the second optically anisotropic layer and the longitudinal direction of the transparent substrate is substantially 15xc2x0, the angle between the slow axis in the plane of the second optically anisotropic layer and the slow axis in the plane of the first optically anisotropic layer is substantially 60xc2x0 and the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the first optically anisotropic layer is substantially orthogonal to the rubbing axis of the first alignment layer.
As preferred, there are provided the retarder comprising a second alignment layer having a rubbing axis defining the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the second optically anisotropic layer, wherein the angle between the rubbing axis of the first alignment layer and the longitudinal direction of the transparent substrate is substantially xe2x88x9215xc2x0 and the angle between the rubbing axis of the second alignment,layer and the longitudinal direction of the transparent substrate is substantially 15xc2x0; the retarder wherein the first alignment layer is formed of at least one copolymer having at least one of repeating units represented by any one of formulae (I) to (III) below and at least one of repeating units represented by formula (IV) below: 
wherein R1 to R3 independently represent a hydrogen atom, a halogen atom or an alkyl group having 1 to 6 carbon atoms; M represents a proton, an alkali metal ion or an ammonium ion; L0 and L1 independently represent a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NR4xe2x80x94, xe2x80x94SO2xe2x80x94, alkylene groups, alkenylene groups, arylene groups and combinations thereof; R4 represents a hydrogen atom or an alkyl group containing 1 to 6 carbon atoms; R0 represents a C10-100 group containing at least two aromatic rings or aromatic heterocycles; Cy represents a condensed aromatic cyclic hydrocarbon group or a condensed aromatic heterocyclic group; Q represents a polymerizable group; and m, n1, n2 and p represent the mol % of each repeating unit where m is 10-99 mol %, n1 and n2 are each 1-90 mol % and p is 0.1-20 mol %; the retarder wherein the first optically anisotropic layer contains at least an agent for promoting homogenous alignment represented by formula (V) below:
(Hbxe2x80x94L2xe2x80x94)nB1xe2x80x83xe2x80x83Formula (V)
wherein Hb represents an aliphatic group having 6 to 40 carbon atoms or an aliphatic-substituted oligosiloxanoxy group having 6 to 40 carbon atoms; L2 represents a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NR5xe2x80x94, xe2x80x94SO2xe2x80x94, alkylene groups, alkenylene groups, arylene groups and combinations thereof; R5 represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; n represents an integer of any of 2 to 12; and B1 represents an n-valent group containing at least three cyclic structures;
and wherein the tilt angle of the rod-like liquid crystal compound is substantially less than 10xc2x0; and the retarder wherein the rod-like liquid crystal compound in the first optically anisotropic layer or the second optically anisotropic layer is at least one of rod-like liquid crystal compounds of formula (VI) below:
Q11xe2x80x94L11xe2x80x94Cy11xe2x80x94L12xe2x80x94(Cy12xe2x80x94L13)nxe2x80x94Cy13xe2x80x94L14xe2x80x94Q12xe2x80x83xe2x80x83Formula (VI)
wherein Q11 and Q12 independently represent a polymerizable group; L11 and L14 independently represent a divalent linking group; L12 and L13 independently represent a single bond or a divalent linking group; Cy11, Cy12 and Cy13 independently represent a divalent cyclic group; and n represents 0, 1 or 2.
In another aspect, the present invention provides a method for preparing a retarder comprising the steps of:
forming a first alignment layer up on a long transparent substrate, said first alignment layer having a rubbing axis at an angle of substantially xe2x88x9215xc2x0 with respect to the longitudinal direction of the transparent substrate,
forming a first optically anisotropic layer by applying a rod-like liquid crystal compound to the first alignment layer and homogenously aligning the rod-like liquid crystal compound in an azimuthal direction substantially orthogonal to the rubbing axis of the first alignment layer, so as that the first optically anisotropic layer has a phase shift substantially equal to n measured at a wavelength of 550 nm,
forming a second alignment layer on the transparent substrate, said second alignment layer having a rubbing axis at an angle of substantially 15xc2x0 with respect to the longitudinal direction of the transparent substrate, and
forming a second optically anisotropic layer by applying a rod-like liquid crystal compound to the second alignment layer and homogenously aligning the rod-like liquid crystal compound in an azimuthal direction substantially parallel to the rubbing axis of the second alignment layer, so as that the second optically anisotropic layer has a phase shift substantially equal to xcfx80/2 measured at a wavelength of 550 nm.
In another aspect, the present invention provides a circular polarizer comprising:
a long transparent substrate having a front surface and rear surface, and up on the front surface of the substrate,
a first optically anisotropic layer formed of a homogenously aligned rod-like liquid crystal compound and having a phase shift substantially equal to n measured at a wavelength of 550 nm;
a second optically anisotropic layer formed of a homogenously aligned rod-like liquid crystal compound and having a phase shift substantially equal to xcfx80/2. measured at a wavelength of 550 nm; and
a first alignment layer having a rubbing axis defining the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the first optically anisotropic layer;
and on the rear surface of the substrate,
a polarizing film having a polarization axis substantially orthogonal to the longitudinal direction of the substrate;
wherein the angle between the slow axis in the plane of the first optically anisotropic layer and the longitudinal direction of the transparent substrate is substantially 75xc2x0, the angle between the slow axis in the plane of the second optically anisotropic layer and the longitudinal direction of the transparent substrate is substantially 15xc2x0, the angle between the slow axis in the plane of the second optically anisotropic layer and the slow axis in the plane of the first optically anisotropic layer is substantially 60xc2x0 and the azimuthal direction of the homogenous alignment of the rod-like liquid crystal compound in the first optically anisotropic layer is substantially orthogonal to the rubbing axis of the first alignment layer.
In another aspect, the present invention provides a method for preparing a circular polarizer comprising the steps of:
forming a first alignment layer up on a front surface of a long transparent substrate, said first alignment layer having a rubbing axis at an angle of substantially xe2x88x9215xc2x0 with respect to the longitudinal direction of the transparent substrate,
forming a first optically anisotropic layer by applying a rod-like liquid crystal compound to the first alignment layer and homogenously aligning the rod-like liquid crystal compound in an azimuthal direction substantially orthogonal to the rubbing axis of the first alignment layer, so as that the first optically anisotropic layer has a phase shift substantially equal to n measured at a wavelength of 550 nm,
forming a second alignment layer on the transparent substrate, said second alignment layer having a rubbing axis at an angle of substantially 15xc2x0 with respect to the longitudinal direction of the transparent substrate,
forming a second optically anisotropic layer by applying a rod-like liquid crystal compound to the second alignment layer and homogenously aligning the rod-like liquid crystal compound in an azimuthal direction substantially parallel to the rubbing axis of the second alignment layer, so as that the second optically anisotropic layer has a phase shift substantially equal to xcfx80/2 measured at a wavelength of 550 nm, and
forming a polarizing film on a rear surface of the substrate, having an absorption axis substantially parallel to the longitudinal direction of the transparent substrate and a polarization axis in a direction substantially perpendicular to the longitudinal direction of the transparent substrate.
As used herein, the expressions xe2x80x9csubstantially 15xc2x0xe2x80x9d, xe2x80x9csubstantially xe2x88x9215xc2x0xe2x80x9d, xe2x80x9csubstantially 75xc2x0xe2x80x9d, xe2x80x9csubstantially 60xc2x0xe2x80x9d, xe2x80x9csubstantially parallelxe2x80x9d and xe2x80x9csubstantially orthogonalxe2x80x9d mean that each angle is within the range of the exact angle less than 5xc2x0. The error from the exact angle is preferably less than 4xc2x0, more preferably less than 3xc2x0. As used herein, the xe2x80x9cslow axisxe2x80x9d means the direction in which the refractive index is maximum and the xe2x80x9crubbing axisxe2x80x9d means the direction of rubbing. Also as used herein, the expressions xe2x80x9ca front surface of a transparent substratexe2x80x9d and xe2x80x9ca rear surface of a transparent substratexe2x80x9d simply mean the directions viewed from the transparent substrate, but should not be construed to limit embodiments in which retarders and circular polarizers of the present invention are used.